The present invention relates to an electrophotographic photoreceptor, in particular, one that is suitable for use with printers, copiers, etc. and which shows high sensitivity to light from LEDS and Laser Diode. Electrophotographic photoreceptors having high sensitivity to visible light are used extensively with copiers, printers, etc.
Most common photoreceptors that are used in these applications are inorganic photoreceptors provided with light-sensitive layers that are chiefly composed of inorganic photoconductive materials such as selenium, zinc oxide and cadmium sulfide. However, such inorganic photoreceptors are not completely satisfactory in such characteristics as photosensitivity, heat stability, moisture resistance and durability that are required of electrophotographic photoreceptors to be used with copiers, printers, etc. For instance, selenium will crystallize upon heating or exposure to dirt such as sebum, often leading to deterioration of the photoreceptors that use it as a photoconductor. Photoreceptors that use cadmium sulfide are low in moisture resistance and durability, whereas those using zinc oxide are poor in durability. Further, photoreceptors using selenium or cadmium sulfide are subject to great restraints during manufacture and handling because of the toxicity of these materials.
With a view to solving these problems with inorganic photoconductive materials, attempts have been made to use a variety of organic photoconductive materials in the light-sensitive layers of photoreceptors and active R&D efforts are being made today along this line. For example, Japanese Patent Publication No. 50-10496 describes an organic photoreceptor having a light-sensitive layer containing polyvinylcarbazole and trinitro-fluorenone. However, even this photoreceptor is not completely satisfactory in terms of sensitivity and durability. To overcome this problem, an electrophotographic photoreceptor of a functionally separated type in which carrier generating and transporting capabilities are individually fulfilled by different materials has been developed. In this type of photoreceptors, suitable materials can be selected from a broad range of choices and hence a photoreceptor having desired characteristics can be obtained fairly easily. For these reasons, it is anticipated that an organic photoreceptor having high sensitivity and durability can be produced using the concept of function separation.
Various organic dyes and pigments have so far been proposed for use as carrier generating materials in functionally separated electrophotographic photoreceptors and those which are used commercially include polycyclic quinone compounds typified by dibromoanthanthrone, pyrylium compounds, eutectic complexes of pyrylium compounds and polycarbonates, squarium compounds, phthalocyanine compounds, azo compounds, etc.
However, many of these carrier generating materials are predominantly sensitive to the short or medium wavelength range of visible light and they are not suitable for use in photoreceptors on laser printers that employ semiconductor lasers as light sources since they do not have the necessary sensitivity in the operating wavelength range of 750-850 nm. Certain azo compounds are phthalocyanine compounds have been found to have predominant sensitivity in a wavelength range longer than 750 nm. These compounds are provided with a specific aggregated or crystalline structure not only to shift the predominant absorption to the longer wavelength range but also to enhance their ability to generate carriers. To design these compounds, reviewing the conditions for their production and those for fabricating photoreceptors is important. Because of these technical complexities, no carrier generating materials have been discovered that are satisfactory in all aspects including chargeability, sensitivity and resistance to cyclic use and an electrophotographic photoreceptor having high performance is yet to be developed.
In ordinary electrophotographic photoreceptors, the electrical contact between a grounded conductive layer and a light-sensitive layer is not microscopically uniform and the efficiency of carrier injection from the conductive layer may differ from one site to another, creating local differences in the distribution of electric charges held on the photoreceptor's surface. These differences will become visible as image defects after development, which are white spots in the black background in a positive-working development process or black spots in the white background in a negative- working reversal development process. In particular, black spots appearing in a reversal development process are as deleterious to image quality as background fogging.
With a view to solving these problems, it has been proposed that an intermediate layer be provided between a conductive layer and a light-sensitive layer so as to block carrier injection into the light-sensitive layer. However, as the blocking characteristic increases, the sensitivity of the photoreceptor will decrease or the residual potential remaining after exposure will increase. Thus, an optimum combination of materials for light-sensitive layer and intermediate layer has not yet been discovered that is capable of blocking carrier injection without suffering the disadvantages described above.